Extracts of taxus species and cell cultures of taxus contain taxol, a promising chemotherapeutic agent which was recently approved by the Food and Drug Administration for the treatment of refractory ovarian cancer. Currently, taxol is isolated on a large scale from the bark of taxus brevifolia. However, taxol can be potentially isolated from other parts of taxus and also from cell culture of taxus. The isolation of taxol from any natural source is complex. A particularly difficult and expensive part of the isolation of taxol is the separation of a closely-related diterpenoid compound called cephalomannine. The structures and chemical properties of taxol and cephalomannine are similar. The only structural difference between the two compounds involves the tail portion of the molecule (see 1 and 2 in FIG. 1). Separation of taxol from cephalomannine is very difficult, but it can be done by means of chromatography. This is an expensive method when performed on a large commercial scale.
Chemical modifications of the tail portion of cephalomannine in the presence of taxol in purified and partially-purified taxane mixtures has been described by Kingston, Journal of Natural Products, Vol. 55, No. 2, pp. 259-261, February, 1992, incorporated herein by reference. In the partially-purified mixture the taxol and the cephalomannine were present in an amount of about 80% by weight. Selective oxidation of the riglate group of cephalomannine using osmium tetroxide under stoichiometric or catalytic conditions (Kingston's method) yielded a diol (compound 3). The diol was separated from the reaction mixture by chromatography. This is shown in FIG. 2 herein.
As explained in more detail hereinafter, the Kingston process is not suitable for use with unpurified taxane mixtures. Osmium tetroxide catalyzed oxidation of cephalomannine is not amenable to impure extracts from biomass that contain taxol because of the strict reaction conditions required (purity of substrates, anhydrous reagents and solvents, etc. ) for good transformation. The use of osmium tetroxide in the bulk manufacture of a pharmaceutical is also undesirable due to its severe toxicity.
There has not heretofore been provided a process for the simple, safe and effective oxidation of cephalomannine in the presence of taxol, nor has there been provided a process for the effective oxidation of cephalomannine in impure mixtures.
Also, there has not heretofore been provided a general, selective, and comprehensive process for oxidation of the olefin functional groups of taxanes to produce oxidized taxane compounds.